This invention relates to fuel injectors in general and particularly to fuel injectors for Compressed Natural Gas (CNG). More particularly, this invention relates to a damping system for counteracting rebound of a valve needle during the operation of a fuel injector.
Compressed natural gas, which is a common fuel for commercial fleet vehicles, is delivered to an engine through one or more fuel injectors. Each injector is required to deliver a precise amount of fuel per injection pulse, and maintain this precision over the life of the injector. In order to maintain this level of performance for an injector, certain strategies and sequences of operations are required to optimize the combustion of the fuel.
In order to promote efficient fuel consumption, the injector is required to open and close very quickly. This is effectively accomplished using a magnetic circuit to displace the valve needle with respect to an injector outlet seat. Specifically, a magnetic fieldxe2x80x94or fluxxe2x80x94is produced relatively quickly across a working gap between a fuel inlet member, which acts as a stator, and an armature connected to the valve needle. A conventional magnetic circuit for an injector includes the inlet member, the armature, a valve body shell, a housing (providing a flux return path), and a coil. When energized, the coil produces the flux that is conducted through the steel parts of the magnetic circuit. The flux creates an attractive (or repulsive) force at the working gap, which moves the armature and valve needle, to open (or close) the injector.
However, quickly opening the injector creates a relatively severe impact between the armature and the inlet member. And quickly closing the injector creates a relatively severe impact between the armature needle assembly and injector outlet seat. In a CNG injector, the factors that affect the injector opening and closing impact velocities are more severe than in a gasoline injector. Compared to the gasoline injector, the CNG injector has two to three times more lift, less spring preload, and a similar force required to open the injector. These factors are exaggerated by the lower viscosity of CNG relative to gasoline.
The much greater lift of the CNG injector corresponds to the need for a much higher flow rate and area in order to obtain the same amount of energy flow through the injector for a given pulse. This is because CNG has a relatively lower density than gasoline.
The increased lift creates two problems. First, the increased lift substantially reduces the magnetic force available to open the injector. Second, the velocities created because of the longer flight times can be higher, creating higher impact momentum. The reduction in magnetic force also creates another problem: it is necessary to use a lighter spring preload than in a gasoline injector.
A conventional gasoline injector uses about four Newtons of spring preload and a very small gasoline force on the needle armature assembly while the injector is closed. In a CNG injector, the force of the gas pressure is about three Newtons and the force of the spring is about two Newtons. In operation, energizing a CNG or gasoline injector causes the needle armature to begin to move when the magnetic force reaches a level that overcomes the spring and the fuel force. However, in a CNG injector, the fuel force is removed as soon as the needle/seat seal is broken and the pressure equalizes at the tip of the needle. At this point, the magnetic force is substantially higher then it needs to be to lift the armature needle assembly against the force of the spring. This excess magnetic force, combined with a relatively light spring preload, high lift, and low viscosity CNG all contribute to high impact velocities between the armature and the inlet member. Lifting the needle also allows CNG to jet out through the injector outlet seat. To close the injector outlet, the magnetic coil is de-energized. In absence of the magnetic force, the armature needle assembly travels under the bias of the spring until the needle tip contacts the injector outlet seat, thereby closing the injector. The high velocity of the armature needle assembly that culminates in the closing impact between the needle tip and the injector outlet seat can cause the armature needle assembly to rebound, which can result in an uncontrolled secondary fuel injection(s). Thus, there is a need to provide fuel injectors (compressed natural gas injectors in particular) with mechanical damping for the armature needle assembly during opening and closing of the gaseous fuel valve.
The present invention provides a valve arrangement for metering fluid flow. The valve arrangement includes a valve seat including an orifice through which fluid flows. The valve arrangement also includes a valve displaceable along an axis between a first position contiguously engaging the valve seat and a second position spaced from the valve seat. Fluid flow between the valve seat and the valve is prevented in the first position and is permitted in the second position. The valve arrangement further includes a counterweight mounted on the valve for relative movement therebetween.
The present invention also provides a fuel injector for metering fuel flow to a combustion chamber of an internal combustion engine. The fuel injector includes a body having an inlet, an outlet, and a fuel flow passage extending along an axis between the inlet and the outlet. The fuel injector further includes a valve seat that is proximate to the outlet and an orifice through which fuel flows. The fuel injector also includes an armature assembly positioned in the passage and displaceable along the axis between first and second positions. The armature assembly includes a valve contiguously engaging the valve seat in the first position to prevent fuel flow through the orifice and spaced from the valve seat in the second position to permit fuel flow through the orifice. The armature assembly also includes a counterweight mounted for relative movement with respect to the armature assembly.
The present invention further provides a method of preventing uncontrolled fuel flow from a fuel injector having an inlet, an outlet, and a fuel flow passage extending along an axis between the inlet and the outlet. The method includes providing a valve seat proximate the outlet. The valve seat includes an orifice through which fuel flows. The method further includes providing an armature assembly displaceable along the axis between first and second positions. The armature assembly includes a valve contiguously engaging the valve seat in the first position to prevent fuel flow through the orifice and being spaced from the valve seat in the second position to permit fuel flow through the orifice. The method also includes mounting a counterweight on the armature assembly for relative movement therebetween.